The East Asian rainy season, commonly called the plum rain (Chinese and Japanese: 梅雨; pinyin: méiyǔ; rōmaji: tsuyu/baiu; Korean: 장마; romaja: jangma; Russian: затяжные, zatyazhnie), is caused by precipitation along a persistent stationary front known as the Mei-Yu front for nearly two months during the late spring and early summer between eastern Russia, China, Korea, and Japan. The wet season ends during the summer when the subtropical ridge becomes strong enough to push this front north of the region.
An east-west zone of disturbed weather during spring along this front stretches from the east China coast, initially across Taiwan and Okinawa, later, when it has shifted to the north, eastward into the southern peninsula of South Korea and Japan. The rainy season usually lasts from May to June in Taiwan and Okinawa, from June to July (approximately 50 days) in Russian Primorsky Krai, Japan and Korea and from July to August in Eastern China (especially the Chang Jiang and Huai River regions).
The weather front forms when the moist air over the Pacific meets the cooler continental air mass. The front and the formation of frontal depressions along it brings precipitation to Primorsky Krai, Japan, Korea, eastern China, and Taiwan. As the front moves back and forth depending on the strength of cool and warm air masses, there is often prolonged precipitation and sometimes flooding in eastern China. However, in the years that it does not rain as much as usual, a drought might result. The rainy season ends when the warm air mass associated with the subtropical ridge is strong enough to push the front north and away.
The high humidity in the air during this season encourages the formation of mold and rot not only on food but on fabrics as well. Environmentally, heavy rains encourage mudslides and flooding in all areas affected. The most rain in a one-hour period as recorded in Japan was in Nagasaki in 1982 with 153 mm. The highest overall recorded rainfall during the rainy season in Japan was in 2003, when Miyazaki Prefecture recorded rains of 8670 mm.
In Japan, the rainy season lasts from early June to mid-July for most of the country (on the main island of Honshū and the islands of Kyūshū and Shikoku), approximately June 7 to July 20 for the main Kansai and Kantō regions. It comes a month earlier to Okinawa in the south (early May through mid-June), but Hokkaidō in the north is largely unaffected. The season is occasionally called Samidare (五月雨, literally "May Rain (in Japanese traditional calendar)"; corresponds roughly to June in modern calendar) on account of this timing. The enka artist Eiichi Ōtaki produced a popular song by this name, and a WW2 Japanese naval ship was also given this name.
The rains in the middle of November - early December are sometimes called the Sazanka Tsuyu, literally "rainy season of the Camellia" on account of the timing with the blossoming of the seasonal flower.
This period is generally avoided for tourism, but some sights are considered particularly atmospheric in the rain and fog, particularly mountain forests, notably Sacred Sites and Pilgrimage Routes in the Kii Mountain Range (including Mount Kōya). Vegetation, especially moss, is also rather lush at this time, and hence sights known for their moss, such as Saihō-ji (the moss temple) are also popular at this time of year.
The rainy season is between June and mid-July. It is caused by hot and humid high pressure forming in the Sea of Okhotsk due to the North Pacific anticyclone combining with Asiatic continental high pressure. When the two meteorological events meet they form a long jangmajeonseon (Hanja: 장마前線). Beginning in late-May, the North Pacific high pressure forces the weaker continental anticyclone south of the island of Okinawa. This fall to the south then reverses and gradually strengthens as it moves northwards back towards the Korean peninsula. On landfall, heavy monsoon rains lead to torrential downpours and flooding. By August the system has weakened as the southern systems retreat towards the Filipino archipelago.
By early autumn, the North Pacific high pressure system is pushed away as Asiatic continental cold high pressure moves southwards. This produces inclement weather although not on the scale of the summer monsoons. Korea can, however, be struck by typhoons during this period.
In some years, the rainy season's actual beginning and end are under debate. For example, in 2005, the subtropical ridge moved quickly northward in late June/early July. The weather front skipped the Chang Jiang region and there was no rainy season there. Then, the ridge retreated southward and there was significant rainfall in the region. This gave rise to the question of whether this was the summer-type rainfall pattern that is common after the first rainy season or the second rainy season. Some meteorologists even argued that the rainy period in late June was not a true rainy season.
-  Archived 2006-04-27 at the Wayback Machine.
- "Okinawa Travel Information".
- Rainy Season (Tsuyu), japan-guide.com
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